CIA director John Brennan on Sunday gave an optimistic assessment of U.S. efforts against the Islamic State, saying that the terrorist group's expansion has been "blunted."

Washington Examiner "Clearly, ISIS momentum inside of Iraq and Syria has been blunted, and it has been stopped," Brennan said in an interview with Fox News, using another name by which the Islamic State is known.

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"They are not on the march as they were several months ago," he said. "And so, our working with the Iraqis and the Iraqis now trying to push back against it, it is having some great, I think, progress."

The U.S. has been engaged in a campaign against the Islamic State using air power and training regional forces to engage Islamic State militants.

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Groups affiliated with the Islamic State, however, have branched into North Africa in recent months, and took responsibility for terrorist attacks this week in Yemen and Tunisia.

Brennan acknowledged those gains by the Islamic State, saying that the U.S. "cannot relent" in working with regional partners against the terrorists.

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But he defended his past comments in 2012 that the U.S. had Al-Qaida on the run, saying that the Islamic State is a new and separate phenomenon.

"There was no sense that I think either I or the president or others gave to the American people that terrorism was going away," Brennan said. "But we've made great progress against a lot of these groups that had plans in place to carry out attacks."

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Despite his assessment that the Islamic State's progress had been stopped, Brennan still placed the group equal to Iran in terms of the threat it posed the U.S.

Earlier this week, former CIA director David Petraeus warned that Iran's influence, not the Islamic State, was the greater threat to stability in the Middle East.

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Brennan stopped short of agreeing with that assessment, and also said that Iran was not an ally against the Islamic State. "I wouldn't say that Iran is an ally right now," he said.

"The Iranians are pursuing their interests inside of Iraq, some of which include efforts against Daesh and preventing that phenomenon from growing," Brennan said, using another name identifying the Islamic State.